A fuel cell is an electrochemical system in which a fuel (such as hydrogen) is reacted with an oxidant (such as oxygen) at high temperature to generate electricity. A fuel cell is typically supported by a system of components such as reformers, heat exchangers, ejectors, combustors, fuel and oxidant sources, and other components. For example, a source of unreformed fuel may be supplied via a fuel ejector to the fuel cell system reformer. The reformer may partially or completely reform the fuel by steam, dry, or other reforming method to produce a reformate that is supplied to the anodes of the fuel cell. The fuel cell may exhaust unused fuel from the anode and supply the unused fuel to either the suction of the fuel ejector or an auxiliary system.
To facilitate the reformation of the unreformed fuel, the fuel cell system may provide a heat input to the reformer by supplying the cathode exhaust, or other hot fluid, to the reformer. After transferring its heat into the reforming fuel, the cathode exhaust may be supplied to an auxiliary system, recycled back to the cathode inlet via an oxidant air ejector, or both.
While the temperature of the recycled and fresh oxidant supplied to the cathode will increase as it passes through the fuel cell stack, the heat input into the cathode flow may be insufficient to maintain the cathode loop in thermal equilibrium given the large heat input needed to support the reformation of the hydrocarbon fuel. To thermally balance the cathode loop, a heat exchanger may be introduced in the cathode loop, typically upstream of the cathode inlet. This heat exchanger may be supplied with the combustion products from the reaction of unused fuel and the cathode exhaust supplied to an auxiliary system. This reaction may occur in the heat exchanger or in a component, such as, e.g., a combustor, upstream of the heat exchanger.
The cathode loop is maintained in thermal equilibrium during normal operations. The heat generated within the fuel cell stack, the heat transferred into the fuel in the reformer, the cooling effect of the oxidant mixing at the cathode ejector, and the heat input from the heat exchanger will balance to maintain this thermal equilibrium; in fact, the heat exchanger upstream of the cathode inlet is sized for just such a purpose.
One type of fuel cell is the solid oxide fuel cell (SOFC). The basic components of a SOFC may include an anode, a cathode, a solid electrolyte, and an interconnect. The fuel may be supplied to the anode, and the oxidant may be supplied to the cathode of the fuel cell. At the cathode, electrons may ionize the oxidant. The electrolyte may comprise a material that allows the ionized oxidant to pass therethrough to the anode while simultaneously being impervious to the fluid fuel and oxidant. At the anode, the fuel is combined with the ionized oxidant in a reaction that releases electrons which are conducted back to the cathode through the interconnect. Heat generated from ohmic losses is removed from the fuel cell by either the anode or cathode exhaust flows or is radiated to the environment. The heat from these electrical loses could be used for the reformation of a hydrocarbon fuel within the fuel cell stack.
A SOFC may be structured, e.g., as a segment-in-series or in-plane series arrangement of individual cells. The oxidant is typically introduced at one end of the series of cells and flows over the remaining cells until reaching the cathode exhaust outlet. Each fuel cell transfers heat into the oxidant thereby raising its temperature and forming a temperature gradient that increases from the oxidant inlet to the exhaust. A temperature gradient may also develop in the fuel cell which increases from the oxidant inlet to the oxidant exhaust. These temperature gradients cause thermal stresses that may cause material degradation or failure of the fuel cell components or may reduce fuel cell performance.
The anode of a SOFC may be a mixed cermet comprising nickel and zirconia (such as, e.g., yttria stabilized zirconia (YSZ)) or nickel and ceria (such as, e.g., gadolinia dope ceria (GDC)). Nickel, and other materials, may function not only to support the chemical reaction between the fuel and the ionized oxidant but may have catalytic properties which allow the anode to reform a hydrocarbon fuel within the fuel cell. One method of reforming the hydrocarbon fuel is steam reforming of methane (CH4), an endothermic reaction (Equation 1):CH4+H2O→CO+3H2ΔH°=206.2 kJ/mole  (Equation 1)Alternative methods of reforming are also available. For example, the hydrocarbon fuel may be reformed by carbon dioxide reforming (also known as dry reforming) (Equation 2):CO2+CH4→2H2+2CO  (Equation 2)
The heat necessary for the reformation of methane could be supplied directly from the heat generated within the stack. This direct heat transfer may help cool the stack, reduce thermal stresses and improve overall stack performance.
Additionally, the direct heat transfer may remove or reduce the amount of heat needed for the reformation of a hydrocarbon fuel in the reformer. The removal of this large heat sink in the cathode loop may allow for a revised fuel cell cycle that improves fuel cell system efficiency while maintaining the cathode loop in thermal equilibrium.
There remains a need for revised fuel cell thermodynamic cycles for fuel cells that are configured for internal block reforming.
In accordance with some embodiments of the present disclosure, a fuel cell cycle is presented. The cycle may maintain the overall thermal balance of the cathode loop. The cycle may not require a heat transfer from the cathode exhaust into the reformer to facilitate a catalytic reformation of unreformed fuel. The fuel, either all or a portion thereof, may be reformed internally by either wet or dry reforming, wherein the heat necessary for the reformation of the unreformed hydrocarbon fuel is transferred from the heat generated with the fuel cell stack. An external reformer may be reduced in size when compared to reformers used in fuel cell cycles in which all or a majority of the fuel is reformed external to the fuel cell block. The heat exchanger upstream of the cathode inlet may be removed. In some embodiments, the fuel cell cycle may not contain an auxiliary loop.
In accordance with some embodiments of the present disclosure, a fuel cell system is provided. The fuel cell system may comprise a source of unreformed fuel and a source of oxidant. The system may further comprise a fuel cell stack, an anode ejector, a reformed, an auxiliary ejector, and a cathode ejector. The fuel cell stack may comprise a plurality of fuel cells each having an anode, cathode and an electrolyte. The fuel cell may be an SOFC. The stack may further comprise a fuel supply manifold configured to receive a reformate and unreformed fuel and to supply the reformate and unreformed fuel to the fuel cell, a fuel exhaust manifold configured to exhaust unused fuel from the fuel cell stack, an oxidant supply manifold configured to receive an oxidant and to supply the oxidant to the fuel cell and an oxidant exhaust manifold configured to exhaust the oxidant from the fuel cell stack. The anode ejector may be configured to receive unreformed fuel from the source of fuel and to receive a portion of the unused fuel exhausted from the fuel cell stack. The reformer may comprise a plurality of cold-side channels and a plurality of hot-side channels, a fuel supply manifold configured to receive fuel from the anode ejector and to supply the fuel to the plurality of cold-side channels, a fuel exhaust manifold configured to exhaust reformate from the plurality of cold-side channels and to supply the reformate to the fuel supply manifold of the fuel cell stack, an oxidant inlet manifold configured to receive a portion of the oxidant exhausted from the fuel cell stack and to supply the oxidant to the plurality of hot-side channels, and an oxidant exhaust manifold configured to exhaust the oxidant from the plurality of hot-side channels. The auxiliary ejector may be configured to receive a portion of the unused fuel exhausted from the fuel cell stack and to receive the oxidant exhausted from the plurality of hot channels. The cathode ejector may be configured to receive oxidant from a compressor and to receive oxidant exhausted from the oxidant exhaust manifold of the fuel cell stack and to supply oxidant to the oxidant inlet manifold of the fuel cell stack. The fuel cell system may further comprise a combustor configured to receive unused fuel and oxidant exhausted from the auxiliary ejector, a turbine configured to receive the exhaust from the combustor, and a compressor configured to receive oxidant from the oxidant source.
In accordance with some embodiments of the present disclosure, a solid oxide fuel cell system is provided. The system may comprise a fuel cell stack, a reformer, an anode loop, a cathode loop, and an auxiliary loop. The solid oxide fuel cell stack may comprise at least one solid oxide fuel cell, each solid oxide fuel cell comprising an anode, a cathode, and an electrolyte. The reformer may comprise hot- and cold-side channels. The anode loop may supply fuel and reformate to the anode of each solid oxide fuel cell, and may comprise a fuel inlet manifold in the fuel cell stack configured to supply fuel and reformate to the anode of each solid oxide fuel cell, a fuel exhaust manifold configured to receive unused fuel from the anode of each solid oxide fuel cell, a source of fuel, an anode ejector configured to receive fuel from the fuel source and the fuel exhaust manifold, the cold-side channels of said reformer configured to receive fuel from said anode ejector. The cathode loop may supply oxidant to the cathode of each solid oxide fuel cell, an may comprise an oxidant inlet manifold in the fuel cell stack configured to supply oxidant to the cathode of each solid oxide fuel cell, an oxidant exhaust manifold in the fuel cell stack configured to receive unused oxidant from each cathode of the solid oxide fuel cells, a source of oxidant, and a cathode ejector configured to receive oxidant from the oxidant source and the oxidant exhaust manifold and configured to supply oxidant to the oxidant inlet manifold. The auxiliary loop may provide for the combusting a portion of the unused fuel from said fuel exhaust manifold and a portion of the unused oxidant from the oxidant exhaust manifold, an may comprise the hot-side channels of the reformer configured to receive a portion of the unused oxidant from the oxidant exhaust manifold, an auxiliary ejector configured to receive the oxidant from the hot-side channels of the reformer, a portion of the oxidant from the oxidant source, and a portion of the unused fuel from the fuel exhaust manifold; and a combustor configured to receive the exhaust from said auxiliary ejector.
In accordance with some embodiments of the present disclosure, a fuel cell system having at least one fuel cell and a cathode loop for recycling a portion of an unused oxidant from the fuel cell for reuse in the same fuel cell is provided. The cathode loop may comprise an oxidant inlet manifold in the fuel cell configured to supply oxidant to the fuel cell, an oxidant exhaust manifold in the fuel cell configured to receive unused oxidant from said fuel cells, and a cathode ejector configured to receive oxidant from an oxidant source and the oxidant exhaust manifold and to supply oxidant to the oxidant inlet manifold, wherein a portion of said unused oxidant is supplied directly to said oxidant inlet manifold from said oxidant exhaust manifold via said cathode ejector.
These and many other advantages of the present subject matter will be readily apparent to one skilled in the art to which the disclosure pertains from a perusal of the claims, the appended drawings, and the following detail description of the embodiments.
Referring to the drawings, some aspects of non-limiting examples of a fuel cell system in accordance with an embodiment of the present disclosure are schematically depicted. In the drawings, various features, components and interrelationships therebetween of aspects of an embodiment of the present disclosure are depicted. However, the present disclosure is not limited to the particular embodiments presented and the components, features and, interrelationships therebetween as are illustrated in the drawings and described herein.